Method of indirect steam generation



July 1; 1 969 R. STROEHLEN 3,452,719

METflOD OF INDIRECT STEAM GENERATION Original Filed Feb. 28, 1967 Sheet 1 of 4 F j Pia/0R mar Lb'ffLer Boiler INVENTOR Richard Szroehlen July 1, 1969 R. STROEHLEN 3,452,719

ETHOD OF INDIRECT STEAM GENERATION Original Filed Feb. 28, 1967 Sheet L of 4 0 (mH'n HEAT EXCHANGER HEAT EXCHANGE/2 INVENTOR h T Fla/7m! Lyme/wen July 1, 1969 R. STROEHLEN 3,452,719

METHOD OF INDIRECT STEAM GENERATION Original Filed Feb. 28, 1967 sheet 3 of 4 410 "c 2 t (m +117 E6 .4 m, t

,{my-mv-m, -m 40 HEAT I 3 'EXCHANGER Richard Szroehlen mvsmon United States Patent 3,452,719 METHOD OF INDIRECT STEAM GENERATION Richard Stroehlen, Nuremberg, Germany, assignor to Maschinenfabrik Augsburg-Nurnberg Aktiengesellschaft, Nuremberg, Germany Original application Feb. 28, 1967, Ser. No. 619,257. Divided and this application Aug. 19, 1968, Ser. No. 753,616 Claims priority, application Germany, Mar. 2, 1966, M 68 591 Int. Cl. F22b 1/08 U.S. Cl. 12231 7 Claims ABSTRACT OF THE DISCLOSURE This application is a division of my copending application Ser. No. 619,257, filed Feb. 28, 1967, for Method of Indirect Steam Generation.

A method of indirect steam generation is known where the supply of the actual heat of evaporation to produce high-pressure superheated steam from highly preheated feed water is effected not by direct transmission of heat from fine gases to the evaporating water in the boiler, but where the boiler serves exclusively to heat saturated steamat a rate constituting a predetermined multiple of the useful steam flow to be produced-40 the desired superheated steam temperature, with the steam flow exceeding the useful steam flow being used as heating steam to heat the feed water to boiling temperature, and to evaporate the feed water, in a steam drunr located outside the boiler. In this method, known as the Lofiler system, the steam drum operates as a direct-contact evaporator in which the heating steam gives off its superheat and leaves the drum together with the saturated steam raised from the feed water. This so-called circulating steam flow is supercharged in a compressor by a pressure differential Which is equal to the pressure losses in the boiler and the pipe system. In other words, there is only one uniform medium, namely steam, to be heated in the boiler whereby difficulties encountered with two-phase mixtures varying in steam quality from O to 100% as in the normal direct steam raising methods are inherently avoided.

In the case of conventional design forced-circulation boilers, it is extremely difiicult to have equal flows in all heating surface tubes connected in parallel. Such unequal distribution tends to result in inhomogeneous steam/water mixtures of varying composition at the outlet of the individual heating surface sections and, consequently, in difficulties to establish stable flow. This, in principle, is eliminated in the systems of indirect evaporation where no stability difliculties exist and definite and predictable conditions exist in the steam generator or nuclear reactor respectively.

A factor of special importance is, furthermore, that the flow rate through the heat-absorbing heating surfaces is a multiple of that in conventional forced-circulation boilers. In this fashion, a greater multiplicity of parallel connected tubes is assured of a positive supply of coolant and, thereby, excessive local heating is positively obviated.

A drawback of indirect steam generation by the abovedescribed method is in the fact that the necessary supercharging of the circulating steam flow involves additional power and that the associated saturated steam compressor constitutes an additional potential source of operating troubles.

The additional power required naturally affects the energy balance considerably and results in high auxiliary power requirements of such systems and, consequently, a tangible reduction of the thermal overall efliciency of power generation.

The task on which the present invention is based is to reduce to a fraction the compressor power required for steam generation according to the Lofiier principle, to obviate the need for extra power entirely in certain cases, or even to produce extra power.

Now, according to the solution offered by this invention, it is proposed, by means of superheated steam m to produce in a heat exchanger from one part m of the feed water flow In steam at such a pressure as is required at the inlet into the superheater, and to pass this steam directly into the superheater. In this manner, the compressor power otherwise required can be eliminated or reduced.

Furthermore, it is proposed to supercharge the heating steam flow after it has cooled down in the heat exchanger by means of a compressor by the amount of pressure loss in the system and, subsequently, to feed it to the direct-contact evaporator from which the saturated steam produced therein flows directly, i.e., without a compressor, into the superheater.

According to a further feature of this invention, the heat exchanger may take the form of a surface evaporator or a counterflow heat exchanger. Likewise, it is possible to connect the heat exchangers in series at the heating steam side.

Moreover, it is proposed by this invention to have part of the feed water, which is not directly fed to the directcontact evaporator, divided into several partial flows having different pressures which are each passed through a separate heat exchanger.

Also covered by this invention is a system where, with the exception of the steam produced in the final pressure stage, the steam produced in each pressure stage is made to drive a turbine from which it is exhausted at the pressure required ahead of the superheater. It is also proposed in this invention that partial flows of the feed water of various pressure stages may be supplied from a common feed pump.

Another feature proposed by this invention is a system where one or several partial flows of the feed water are supplied, after being discharged from the feed water pump, via feed heaters to the heat exchangers, i.e., a surface evaporator and a counterfiow heat exchanger or direct-contact evaporator respectively. Finally, it is proposed by this invention that the turbines of the various pressure stages may be coupled in tandem with the compressor.

The means by which the objects of this invention are obtained are described more fully with reference to the accompanying drawings which show several examples of both the prior art and the method according to the pres ent invention in schematic form, and in which:

FIG. 1 is a circuit diagram of the conventional prior art Lofiier boiler;

FIGS. 2 and 3 each show, respectively, a simple circuit diagram of this invention; and

FIGS. 4 and 5 show, respectively, two further embodiments of the methods covered by this invention.

In the Loffler boiler shown schematically in FIG. 1 which represents the prior art, the feed water flow In admitted through pipe 1 is, ignoring blowdown amounts, equal to the useful steam ml discharged in pipe 2 for any consumer installations and is caused to evaporate in a direct-contact evaporator 3, designed in the form of a drum, by the heating steam flow m admitted to the latter through the pipe 4; the steam so produced is supercharged by the steam compressor 5, the amount of pressure rise being equivalent to the pressure loss occurring through the whole system. The heating steam supplied to the direct-contact evaporator 3 gives off its superheat in it and leaves it also as saturated steam. In other Words, the boiler proper 6, in which the primary energy is released in the form of heat, is supplied with the sum of the steam flow m and the useful steam flow m which is superheated there from the state at the outlet of the steam compressorsomewhat above the saturated steam stateto the desired superheat temperature of the useful steam. Downstream of the boiler 6, the steam flow is divided into the actual useful steam flow m and the heating steam flow m fed into the direct-contact evaporator 3. An electric motor 7 is provided to drive the steam compressor 5.

For an evaluation of this conventional steam generating method there are two criteria which are of considerable importance, viz.

(a) The circulating ratio u which is the ratio of the steam flow passing through the steam compressor (In-1411 to the useful steam flow In. The circulating ratio u therefore indicates what multiple of the useful steam flow m has to be supercharged by the compressor and flows through the boiler proper.

(b) The specific power for the drive of the compressor by the electric motor 7, i.e., a measure for the Work (kwh.) that has to be expended to produce one ton of useful steam.

Simple power considerations on the basis of the energy balances show that both the circulating ratio u and the specific power for generating the useful steam in kwh./ ton are reduced to a minimum if the feed water temperature ahead of the direct-contact evaporator drum is equal to the saturation temperature in the direct-contact evaporator drum.

For the optimum case of preheating the feed Water to saturation temperature, e.g., by multistage regenerative feed heating, the characteristic values for the Lofller boiler, assuming Percent A pressure 105s in the system of A compressor efficiency of 80 and A motor efliciency of 95 are as follows:

100 160 200 220 xg.-f./cm. kg.-f./cm. kg.-f./em. kg.-f./cm. Useful steam condition 500 0. 520 0. 530 0. 530 C.

Circulating ratio a 3. 03 2. 22 1.70 1. 33 Specific power, kwhJ ton 1s. 4 13. 6 9. a

Kg.-i./em. =one atmosphere.

As the heat of evaporation of steam is substantially reduced as the critical pressure is approached, the necessary heating steam fiow decreases in the same proportion and, consequently, also the specific power.

In FIGS. 2 to 5 showing this invention, the numeral 1 again indicates the feed water supply pipe, 2 the pipe for the useful steam flow m, 3 the direct-contact evaporator, 4 the supply pipe for the heating steam flow m 5 the compressor, 6 the superheater, and 7 the electric motor for the drive of the compressor. The superheater 6 may also take the form of a nuclear reactor.

The numeral 8 designates a heat exchanger, or one of the heat exchangers, whereas 9 indicates a pressure-reducing valve. Th feed water flow m is divid d i to he qu ntity m which is evaporated in the heat exchanger 8 and into the remaining feed water flow (m -m which is supplied via the pressure-reducing valve 9 to the direct-contact evaporator 3 to be evaporated there. In other words, the feed water flow m taken to the direct-contact evaporator 3 is reduced by the amount m which is delivered to the heat exchanger 8. This partial flow m is available at a higher pressure so that its pressure need not be increased which is an essential criterion of this invention.

Furthermore, the steam flow (mm +m in the compressor 5 which is driven by the electric motor 7 is supercharged to the pressure at the admission of the steam flow (m+m into the superheater or nuclear reactor 6 (FIG. 2). It is also within the scope of the present invention to supercharge the substantially smaller heating steam flow m to the necessary initial pressure ahead of the superheater or nuclear reactor 6 whereby the throttling of the feed water flow (in-m is eliminated.

According to the circuit shown in FIG. 3, the heating steam m after cooling down in the surface heat exchanger 8 is supercharged in the compressor 5 and, subsequently, conveyed to the direct-contact evaporator 3.

FIGS. 4 and 5 show a few practical embodiments.

In FIG. 4, steam supplied at 490 C. via the pipe 4 is cooled in a counterfiow heat exchanger 11 to about 410 C. The counterfiow heat exchanger has a feed water partial flow m admitted to it which is supercharged to 350 kg.-f./cm. i.e., to supercritical pressure, by a feed pump 16 and heated to 450. The partial flow 121 in the present case 33.7% of the feed water flow, is expanded from 350 kg.-f./cm. to 166 l g.-f./cm. and made to perform work in a turbine 12 and is delivered at this pressure to the superheater or nuclear reactor 6. Shaft 10 connects turbine 12 to compressor 5.

A further feed water partial flow m which is supercharged to the pressure at the inlet to the superheater 6, in this case to 166 kg.-f./cm. by the feed pump 16, is taken to, and evaporated in, the heat exchanger 8. The heating steam flow 111;; is cooled from about 410 C. to 363 C. in the process, supercharged to 166 kg.-f./cm. in the compressor 5, and, eventually, led to the directcontact evaporator 3 in which the balance feed water flow (m-m m is evaporated. Assuming a very conservative efficiency of the turbine 12 of 50% and a moderate efliciency of the compressor 5 of there will be an additional power demand for the turboset which has to be met by the electric motor 7. With a motor efficiency of 96%, the specific power required is as low as 7.15 kwh./ ton useful steam compared with 18.4 kwh./ton in the case of the conventional Lofller boiler. The power savings are therefore quite substantial and would be even higher with better machine efficiencies. The circulating ratio is u=2.48.

The feed water heaters 13, 14 and 15 in FIG. 4 serve to preheat the feed water partial flows preferably to boiling temperature and they can be integrated structurally and flow-wise into a single unit.

The various feed water partial flows may be supercharged to the necessary pressure in the common feed pump 16, it being possible, by an additional group of stages, to supercharge the partial flow m to supercritical pressure.

FIG. 5 shows a further circuit arrangement where no power at all is required for the generation of useful steam. Further essential elements provided are the pressure booster feed pump 17 and the tandem-coupled turbine 18. Compensation in the energy balance for the power consumed by turbine 12, the pressure booster feed pump 17, and the compressor 5 is eifected by the tandem-coupled turbine 18 which is operated by the useful steam flow m. In this case, the pressure and temperature at the outlet of the superheater or nuclear reactor 6 is proposed to be rated so that the specified live steam condition is obtained at the outlet from the turbine 18. Motor 7 is not used except under special circumstances.

Having now described the means by which the objects of the invention are obtained, I claim:

1. In a method of indirect steam generation comprising introducing feed Water and heating steam into a direct-contact evaporator to produce saturated steam, superheating the saturated steam, dividing the superheated steam into useful steam and heating steam and then sending the heating steam to the evaporator, the improvement further comprising heating a partial portion of the feed water with said heating steam in a heat exchanger located between the superheater and a line leading from said evaporator to the superheater to produce feed water steam at a pressure necessary for entry into the superheater, and then introducing this feed water steam into the superheater.

2. In a method as in claim 1, further comprising compressing the heating steam cooled in said heat exchanger (8), and introducing the compressed heating steam into said evaporator.

3. In a method as in claim 2, further comprising subdividing said partial portion of the feed water into several flows of varying pressure, and heating each flow in a separate heat exchanger (8, 11).

4. In a method as in claim 3, said heat exchanger (8) comprising a surface evaporator.

5. In a method as in claim 4, driving a turbine (12) with one of the varying pressure flows and from which the exhaust flow is at the pressure required for introduction into the superheater.

6. In a method as in claim 5, further comprising producing said several flows in a common feed pump (16), and compressing the steam cooled in said heat exchanger (8) in a compressor (5) driven by said turbine (12) through a shaft (10).

7. In a method as in claim 6, further comprising preheating said partial portion of the feed water into several flows in a plurality of preheaters (13, 14, 15) before introduction into the evaporator (3) and the heat exchangers (8, 11), respectively.

References Cited UNITED STATES PATENTS 2,202,507 5/1940 Swietochowski 122-31 3,139,068 6/1964 Blaskowski 122-31 KENNETH W. SPRAGUE, Primary Examiner.

U.S. Cl. X.R. 122-32 

